Ion implantation and/or ion bombardment is of growing interest in polymer science and engineering because of its demonstrated capability to modify the molecular structure, surface morphology and physical properties of polymers. During ion bombardment of polymers in vacuum at a wide range of conditions, the most common are the processes of polymer cross-linking or chain destruction due to energy transfer at atomic collisions and with following volatile final products release from the surface of the polymer, surface carbon content increase, called surface carbonization, and subsequent surface reconstruction. Changes in the index of refraction, optical transmission and reflection, and other optical properties of polymer films, as well as adhesion enhancement of coatings have been shown to follow ion implantation and ion bombardment of polymeric surface(s). Those are typically of significant impact, especially when used in space applications such as on spacecrafts, in order to control the mechanical and thermal optical performances of the material or the equipment on board. There may be a significant increase in density as a result of volume and density changes due to surface carbonization, accompanying ion implantation of polymers, mechanical, optical/electrical properties change (such as surface hardness, wear resistance, oxidation resistance, electrical conductivity). Some research studies were done to identify the mechanisms of ion beams interaction with polymeric surfaces and properties change, such as improvement of adhesion of polymers to metals or metals to polymers, etc. Some patents also discloses some work on ion implantation/ion bombardment on polymeric surfaces, such as U.S. Pat. No. 4,199,650 to Mirtich et al. granted on Apr. 22, 1980, U.S. Pat. No. 4,957,602 to Binder et al. granted on Sep. 18, 1990, U.S. Pat. No. 5,130,161 to Mansur et al. granted on Jul. 14, 1992, U.S. Pat. No. 6,248,409 to Kim granted on Jun. 19, 2001, U.S. Pat. No. 6,787,441 to Koh et al. granted on Sep. 7, 2004, and U.S. Pat. No. 7,309,405 to Cho et al. granted on Dec. 18, 2007.
However, none of the existing prior art discloses nor even suggests any studies made to develop a method of ion beam treatment of thin dielectric polymer films, and a product made by this method, to provide ESD (electrostatic discharge) protection while minimize impact on RF performance, for example, for space antenna sunshields or for any other relevant space applications. In this regard, it is important to mention that surface carbonization of polymers by ion beams or, in short, “surface carbonization”, is a very wide phenomenon and associated term. It covers a wide range of degree(s) of carbonization, from few percent of exceeding carbon concentration up to almost full “graphitization”, with significantly variable amount of chemical bonding reconstruction, such as from sp2 to sp3 carbon bonding states, in polymeric surfaces and subsurface regions, depending on a variety of ion bombardment conditions. The treated surface layers may be very different in composition, structural specifics and final properties. Furthermore, due to variable degrees of carbonization, various thickness of the modified subsurface layers, final change of elemental chemical composition and chemical bonding re-structuring, also depends on the temperature during bombardment and can be identified by advanced surface analysis methods. Therefore final functional properties of the treated surface, such as surface resistivity. RF transparency, and radiation resistance can vary drastically. It is therefore essential to measure and control the desired functional properties (RF transparency, surface resistivity, durability in space environment, etc.) in order to find a special way of treating the material and to define the proper process parameters to get the desired properties. These properties are not inherent to the general process of ion beam treatment of dielectric polymers (or surface carbonization) and cannot be predicted by one skilled in the art without significant research and trials in order to develop a new surface treatment method.
When antenna applications in space are considered and that a dielectric film is required in the RF field (for example sunshields in front of the radiating element and/or reflector of communication antennas), the material needs to fulfill few specific and, to some extends, conflicting requirements. It needs to be RF transparent, or permeable as much as possible, to prevent signal losses, have good thermo-optical properties to control the temperature excursions of the antenna equipment, and have a charge dissipative surface in the entire space-related temperature range to prevent charging and arcing of polymer films under space radiation environment. It means that electrical surface resistivity (SR) over the entire temperature range should be kept within about 105 to 1010 ohms/square; SR to be above about 105-106 ohms/sq. for RF transparency and below 109-1010 ohms/sq. to avoid ESD (electrostatic discharge) issues. This demonstrates the importance to have as low as possible temperature dependence of the surface resistivity to dissipate electrical charges without disturbing RF performance. It is also required to ensure that these properties do not degrade too much over time when those materials are exposed for years in a specified space environment, for instance, such as geosynchronous Earth orbit (GEO) space environment, that includes UV (ultraviolet), ionizing radiations, i.e. energetic protons and electrons, and thermal cycling in vacuum.
There are different ways of providing ESD (electrostatic discharge) protection to surfaces of dielectric-type materials in order to prevent charge buildups followed by damaging discharges on electrically sensitive surfaces, especially when dealing with active components such as antennas, electronics and the like, in space applications.
One of the ways used is to apply semiconductor-based thin coatings, such as silicon (Si) or germanium (Ge) under vacuum deposition processes, on the required surfaces. Such coatings have a tendency to provide for a significantly varying surface resistivity over space-related temperature ranges, from about −150° C. to about +150° C., as can be encountered in space applications, with a generally too high SR at low end temperatures to achieve proper ESD protection. Furthermore, such coatings are known to be fragile or brittle (not robust), thus requiring careful handling, and may be sensitive to humidity level (mostly germanium).
Another known way is the application of an electrically conductive coating, such as indium-tin oxide (ITO), as in U.S. Pat. No. 5,283,592 granted on Feb. 1, 1994 to Bogorad et al. for an “Antenna Sunshield Membrane”. Disadvantages of this ITO coating is that, beside that it is also fragile (susceptible to cracking), it is too electrically conductive to be considered when RF transparency (or semi-transparency) is needed for a space antenna sunshield, especially in modern high-frequency applications or the like, as it behaves as a barrier to RF signals.
Another way of decreasing the SR of dielectric materials is to load the material with electrically conductive particles such as carbon or the like, as in U.S. Pat. No. 6,139,943 granted on Oct. 31, 2001 to Long et al. for a “Black Thermal Control Film and Thermally Controlled Microwave Device Containing Porous Carbon Pigments”. This loading of particles into the material significantly affects its mechanical and thermo-optical properties, as well as its RF transparency in high frequencies, which considerably limit and essentially hinder its use in most modern space antenna applications.
Early sunshield consisted of Kapton™ dielectric sheet painted white, but the properties degraded over time on-orbit, decreasing thermal protection, and increasing RF signal loss. For ITO-coated white paint on black Kapton™ film and ITO-coated clear Kapton™ film with white paint on the second surface, RF losses in the frequency range 2.5 to 15 GHz were known to be on the order of 0.2 dB (decibel), which was not acceptable for operation with current high power signals requirements at Ku-band frequencies and above.
U.S. Pat. No. 5,373,305 granted on Dec. 13, 1994 to Lepore, Jr. et al. offers as an improved sunshield a pigmented flexible film of 0.0005 to 0.003 inch thick with germanium thin coating, vacuum deposited on the space-facing side. Black-pigmented polyimide substrate (KaptonC™ pigmented with carbon black) was preferred, as solar transmittance is virtually zero. The RF loss for uncoated polyimide or polyetherimide film is quoted as being less than 0.02 dB over the 2.5 to 15 GHz frequency range. The proposed black polyimide membrane sunshield construction adds another 0.03 dB for an RF loss of up to 0.05 dB at 15 GHz. Increased loss is expected when using carbon black for pigmentation. Moreover, the electrical conductivity of germanium (and the like semi-conductor coatings such as silicon) decreases at cold yielding to inadequate ESD protection at cold temperature and increases at hot temperatures yielding to higher RF losses and even possibly to a thermal runaway under high RF power signal densities travelling there through. This type of sunshield is therefore not promising for high-power and/or high-frequency operation, particularly in and above Ku-band and Ka-band frequencies, used in nowadays modern applications.
Accordingly, there is a need for an improved charge dissipative surface of a dielectric polymeric materials, such as, for example, space polymer films, with low temperature dependence of surface resistivity while keeping unchanged RF performance thereof, and a method of making that surface on space polymers.